This invention relates to lasers and, more particularly, to a method and apparatus for producing eyesafe high-peak-power laser radiation from a diode-pumped solid-state laser.
Diode-pumped solid-state lasers are generally considered the most practical source of laser radiation for applications requiring high efficiency and compact, low-weight, and rugged packaging. Laser diode pump sources have high electrical-to-optical conversion efficiency, and the narrow-band spectral output of laser diodes can be chosen to closely match the absorption bands of solid-state laser materials. As a result, heat loads in diode-pumped solid-state lasers are significantly lower than for the flashlamp-pumped solid-state lasers that have been largely supplanted by diode-pumped lasers for applications requiring compactness and high efficiency.
For many applications the laser radiation must be in the xe2x80x9ceyesafexe2x80x9d band, that is having a wavelength longer than 1.4 microns. Laser radiation in this range is strongly absorbed by liquid water. As the fluid in the human eye consists primarily of water, laser radiation in this range does not damage the retina because it is absorbed before reaching the retina.
Several solid-state lasers operate in the eyesafe band, have absorption features suitable for diode pumping, and have millisecond duration fluorescence lifetimes. These lasers are doped with trivalent rare-earth ions and are well-known. An advantage of these lasers is that their long fluorescence lifetimes (i.e. energy storage times) readily allow the production of high-energy Q-switched pulses, and their relatively low emission cross-sections result in pulsewidths that are hundreds of nanoseconds in duration. These pulsewidths are ideal for active remote sensing instrumentation such as coherent detection laser radar systems. Examples of these lasers include:
1) the 1.53-micron ytterbium/erbium co-doped glass (Yb,Er:glass) laser
2) the 2.05-micron thulium/holmium co-doped yttrium lithium fluoride (Tm,Ho:YLF) laser
3) the 2.01-micron thulium-doped yttrium aluminum garnet (Tm:YAG) laser
4) the upper-state-pumped 2.10-micron Ho:YAG laser
5) the upper-state-pumped 1.6-micron erbium-doped bulk crystal (for example, Er:YAG) laser.
Existing eyesafe diode-pumped solid-state lasers have deficiencies that make them non-ideal for critical applications such as remote sensing.
The 1.53-micron Yb,Er:glass laser does not perform well at high average power owing to the poor thermo-mechanical properties of the glass host material. This limits operation of the laser to low pulse repetition frequencies (PRF), thereby severely limiting the sensitivity of remote sensing systems based on this laser.
In the 2.05-micron Tm,Ho:YLF laser, the Tm ion absorbs the pump light and transfers the excitation energy to the lasant Ho ion. This laser suffers from upconversion loss from the upper laser state between pairs of Tm and Ho ions, which reduces the efficiency and energy storage capacity of the laser medium. Energy storage is an important consideration for applications such as remote sensing (where high energy laser pulses enable long-range sensing), because a long energy storage lifetime is required to convert the intrinsically continuous wave (cw) or quasi-cw laser diode pump power into high peak power. In typical Tm,Ho:YLF lasers the energy storage time is reduced from an intrinsic lifetime of 10 ms by upconversion to a lifetime of approximately 1 ms.
The 2.01-micron Tm:YAG laser similarly suffers from upconversion loss, in this case between pairs of Tm ions in the upper laser state. As a result the energy storage lifetime in a typical Tm:YAG laser is reduced from an intrinsic lifetime of 8 ms by upconversion to approximately 3 ms.
The detrimental effects of upconversion loss can be eliminated by pumping the Ho:YAG laser directly into its upper laser state. In this case a low concentration of Ho can be utilized, which suppresses the concentration-dependent upconversion process. The Ho:YAG laser can not be operated efficiently when pumped directly into the upper state by laser diodes, however, because presently there are no laser diodes with sufficiently high power and brightness in the wavelength range (1.85 to 1.95 microns) of strong absorption for efficient upper-state pumping of Ho:YAG. Instead, the Ho:YAG laser must be pumped by an intermediate solid-state laser, for example the 1.94-micron Tm:YALO laser or the 1.91-micron Tm:YLF laser. These lasers have sufficient brightness for pumping the Ho:YAG laser, but have the disadvantage of requiring liquid coolants.
The upper-state-pumped 1.6-micron Er-doped bulk crystal laser operates in a similar fashion to that of the upper-state-pumped Ho:YAG laser. In the case of the Er-doped laser strong absorption for upper-state pumping occurs at 1.533 microns. As in the case of the Ho:YAG laser, laser diodes do not exist presently with sufficiently high power and brightness for efficient upper-state pumping of the Er-doped laser.
A need has existed for many years for efficient eyesafe lasers suitable for remote sensing applications. Such devices are required to operate in the Q-switched mode with high pulse energy and long pulsewidth (hundreds of nanoseconds). In recent years development efforts for such devices have concentrated on solid-state lasers operating at wavelengths near 2 microns. The 2.01-micron Tm:YAG laser and the 2.05-micron Tm,Ho:YLF laser (and variations on these such as the 2.02-micron Tm:LuAG laser) are generally considered to be the most practical sources of efficient, eyesafe, high-energy, long-pulsewidth, Q-switched laser radiation.
In particular, the upper-state pumped 1.6-micron Er-doped bulk crystal laser has been considered to be impractical owing to the lack of a suitable pump source. The 1.6-micron Er-doped laser is known to operate with two pump sources: 1) flashlamp-pumped Yb,Er:glass lasers, and 2) 1.5-micron InGaAs laser diodes. Neither of these pump devices provides output radiation that is suitable for efficient operation of the 1.6-micron Er-doped laser. High-power Yb,Er-doped, Raman-shifted Yb-doped and Raman-shifted Nd-doped fiber lasers have been commercially available for several years; however, these devices have not been previously proposed as pump sources for the upper-state pumped 1.6-micron Er-doped laser. Use of guided-wave lasers, such as fiber lasers (typically having a doped core of circular cross section) and bulk waveguide lasers (typically having a doped region of rectangular cross section), as the pump source is an aspect of the subject invention. In a guided-wave laser the laser radiation is confined to the laser medium (which occupies the same space as the laser resonator) by total internal reflection from the transverse surfaces of the guided-wave structure. The guided-wave geometry is in contrast to conventional lasers in which the laser resonator is defined by discrete mirrors at the ends of a free-space resonator. Guided-wave lasers have the advantage of a small resonator cross section that results in a low laser threshold and efficient removal of heat from the laser medium. An aspect of the present invention is that the pump source of the Er-doped bulk crystal laser is either a Yb,Er-doped guided-wave laser, a Raman-shifted Yb-doped guided-wave laser, or a Raman-shifted Nd-doped guided-wave laser.
A suitable pump laser for the Er-doped laser must meet the following requirements: 1) the pump laser must have sufficiently high electrical-to-optical conversion efficiency, 2) the pump laser must be of small size and low weight, 3) the pump wavelength must be strongly absorbed by the Er-doped laser material, 4) the pump brightness must be sufficiently high for efficient pumping of the Er-doped laser medium, and 5) the pump linewidth must be sufficiently narrow for efficient pumping of the Er-doped laser medium. The flashlamp-pumped Yb,Er:glass pump laser does not meet requirements 1 and 2, while the InGaAs pump laser does not meet requirements 4 and 5.
The flashlamp-pumped Yb,Er:glass laser is known to be impractical as a pump source, because of the large size and weight, and low efficiency of these devices, and also because of the short operational lifetime of flashlamps. Therefore, investigations into the use of the flashlamp-pumped Yb,Er:glass laser as a pump source for the Er-doped bulk crystal laser are considered to be a precursor for direct pumping of the Er-doped laser with 1.5-micron InGaAs laser diodes. However, owing to the unconventional nature of the 1.6-micron Er-doped laser, it is not generally recognized that direct pumping with 1.5-micron laser diodes imposes serious limitations on the performance of the Er-doped laser. Such limitations include: 1) 1.5-micron laser diodes have insufficient brightness for pumping the Er-doped laser, owing to the necessity of avoiding upconversion loss in the Er-doped laser material by utilizing a low Er doping concentration which requires the use of a laser rod having a length greater than the depth of focus of the laser diode pump light, and 2) 1.5-micron laser diodes typically have linewidths greater than 5 nm, making them unsuitable for efficient absorption by the Er-doped laser medium which has a substantially narrower absorption linewidth of approximately 1 nm.
Examples of the use of the flashlamp-pumped Yb,Er:glass laser to pump the 1.6 micron Er:YAG laser are given in two papers by K. Spariosu and M. Bimbaum: 1) in xe2x80x9cIEEE Journal of Quantum Electronics,xe2x80x9d Volume 30, pages 1044-1049, April, 1994, and 2) in OSA Proceedings on Advanced Solid-State Lasers, Volume 13, pages 127-130, 1992.
Thus, there is a need in the art for an efficiently operated upper state pumped 1.6 micron Er-doped laser. The present invention meets these and other needs in the art
It is an aspect of the present invention to produce eyesafe laser radiation from an Er-doped bulk crystal laser device by pumping the Er-doped laser device with a guided-wave laser device, for example a circular-core fiber laser or a waveguide laser (such as a waveguide with rectangular cross section).
It is another aspect of the invention to operate the laser in the Q-switched mode.
Another aspect of the invention is to provide high peak power output from the laser device.
The above and other aspects of the invention are accomplished in a method and apparatus for producing laser radiation with high peak power at an eyesafe wavelength from a diode-pumped solid-state laser. The Er-doped laser is a well-known source of laser radiation operating at an eyesafe wavelength of 1.6 microns. The method and apparatus uses a cw 0.9 to 1.0 micron wavelength laser diode to pump a Yb,Er-doped fiber laser or a Raman-shifted Yb-doped fiber laser or a Raman-shifted Nd-doped fiber laser at a wavelength in the range 1.45 microns to 1.54 microns. The fiber laser further pumps the Er-doped laser to produce high peak power at an eyesafe wavelength of 1.6 microns.
The above and other aspects, features, and advantages of the invention will be better understood by reading the following more particular description of the invention, presented in conjunction with the following drawings, wherein:
FIG. 1 shows a block diagram of the invention;
FIG. 2 shows the impact of beam quality of the pump source for the 1.6-micron Er-doped laser;
FIG. 3 shows the absorption spectrum for Er:YAG material;
FIG. 4 shows the Er:YAG absorption spectrum with an expanded wavelength scale;
FIG. 5 shows a schematic diagram of a one embodiment of the upper-state pumped 1.6-micron Er-doped laser;
FIG. 6 shows a schematic diagram of a test configuration of the upper-state pumped Er:YAG laser;
FIG. 7 shows continuous wave performance of the upper-state pumped Er:YAG laser for a laser rod temperature of xe2x88x9240xc2x0 C.;
FIG. 8 shows the output from the 1.6-micron Er:YAG laser in the Q-switched mode plotted as average power;
FIG. 9 shows the same data as shows in FIG. 8, but plotted terms of pulse energy.